This invention relates to projection color television (TV) display devices using monochrome cathode ray tubes (CRTs) incorporating interference filters, and more particularly relates to a method for improving the white field uniformity of these devices, and to the resulting devices and tubes.
Monochrome cathode ray tubes for projection television employ a single electron gun mounted in the neck of the tube to focus a single electron beam on the fluorescent display screen of the tube. A deflection yoke surrounding the neck of the tube, and associated electronic circuitry, cause the beam to scan the screen as well as to vary n intensity in response to a video signal to produce a monochrome display image.
In projection color television, three such displays, each in one of the primary colors red, blue and green, are superimposed on a large projection screen to produce a full color display image. Because the images on the individual tube screens are not viewed directly, but are magnified and projected by a system of projection lenses, the individual cathode ray tubes are driven at higher loads than would be encountered for direct view tubes, in order to produce a full color display of acceptable brightness.
Projection tubes having an interference filter are described in U.S. Pat. No. 4,634,926, assigned to U.S. Philips Corporation. The filter, herein referred to as a short wave pass (SWP) filter, is composed of alternating layers of materials of high and low refractive index. The filter is designed to result in a marked increase in luminous efficiency of the tube in the forward direction, as well as improved chromaticity and contrast. Even further improvements are provided, especially in light gain in the corners of the display screen, by combining such an interference filter with an inwardly curved display window, as provided in U.S. Pat. No. 4,683,398, also assigned to U.S. Philips Corporation.
In these tubes, the interference filter is deposited directly upon the inner surface of the faceplate and the luminescent phosphor screen is deposited on the interference filter. The filter is typically composed of from 14 to 20 layers each having a thickness of about one-quarter of the central wavelength of the filter.
Such tubes with interference filters, while exhibiting a marked increase in luminous efficiency in the forward direction, as well as improved chromaticity and contrast, also exhibit greater luminance in the center than at the edges of the display, referred to herein as center-to-edge luminance gradient.
Furthermore, uncontrollable variations in the thicknesses of the filters occurring during mass production of these tubes can result in edge luminances which vary from as low as about 30 to as high as about 70 percent of the center luminance. When the red, blue and green tubes are assembled into a projection color television, the tube drivers are adjusted to result in a desired white field at the center of the screen. It can be appreciated that when red, blue and green tubes having different center-to-edge gradients are combined in a projection set, white field uniformity is increasingly degraded as the distance from the center of the screen increases.